I have a pretty good friend, actually, who's one of these people. He has remarkable charisma, and I've never met a single person (this is on a college campus--and he's really outgoing, of course) who wasn't immediately taken with him. He knows it, too... but not in an arrogant way. It's the kind of thing where everyone loves him, and their ideas of things are manipulated by his opinions of people and ideas even when there's no conceivable way that should ever make a difference. He wonders why this is, and sometimes even shifts the way he acts towards things without a reason just to try to figure out what makes people change the way they see things with him--we became friends, I think, largely because I was one of the only people (the only one at college... maybe ever) who wasn't influenced so heavily just by his being around. He strikes me as exactly the kind of person you're thinking about... just by the way he acts and talks and how people respond to it. ENFP, if you're wondering.
That said... self-confidence has nothing to do with it. He, like everyone else with any depth to them, is torn up inside by the same kinds of things everyone else is. You'd just never guess it because it never touches him when he's around other people (save late night, private conversations when no one else is around). I don't think that the nearly-hypnotic allure of such people has anything to do with actual self-confidence, which I don't think anyone but the really narrow-minded have... but it's more of a sort of internal hypocrisy. But I wouldn't say it's purposeful hypocrisy... it just seems like whenever he's around other people, he's consumed by overwhelming happiness and excitement which overflows from within him, regardless of how sad and distressed he was 2 seconds before someone else walked into the room.
I'm thinking that the key to the infectious cheer is the ability to just forget and ignore everything that's bothering you, instantly and without effort. I'm not sure INTPs have that ability, because we've subconsciously chosen truth over making people happy. It's just not what our brains are good at. I'm not really sure, though, because I've only been able to study this guy for 6-7 months or so, and I didn't even really know him til the last 3 or so... so my opinions on the subject are shifting pretty frequenly with experiences.